1. Field of the Invention
The present invention relates to a color television receiver. More specifically, the present invention relates to a novel television receiver wherein the phases of a luminance signal and a chrominance signal are made consistent with each other without employing a delay line for a luminance signal in a video signal processing circuit.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the outline of a video signal processing circuit in a conventional color television receiver wherein the present invention can be advantageously employed. Referring to FIG. 1, a color television signal obtained from a transmitter end, not shown, is applied to a tuner 1 through an antenna. A tuning stage is selectively established to a desired broadcasting signal in the tuner 1, whereby the desired broadcasting signal is converted into a video intermediate frequency signal. The video intermediate frequency signal obtained from the tuner 1 is applied to a video intermediate frequency circuit 2. Although not shown, the video intermediate frequency circuit 2 comprises a video intermediate frequency amplifier, a video detector and the like and the output therefrom is withdrawn as a video signal. The video signal obtained from the video intermediate frequency circuit 2 is applied to a video buffer amplifier 3 constituted by an impedance converting means such as an emitter follower. The video signal obtained from the video buffer amplifier 3 is applied to a luminance signal circuit 4 and a chrominance signal circuit 7. The luminance signal circuit 4 comprises a delay line 5 for delaying the luminance signal separated from the video signal by a predetermined delay time and a luminance signal amplifier 6. On the other hand, the chrominance signal circuit 7 comprises a bandpass filter 8 for extracting a chrominance signal component from the video signal, so that the chrominance signal extracted by the bandpass filter 8 is applied to a chrominance signal processing circuit 9. The chrominance signal processing circuit 9 comprises a bandpass amplifier, a chroma synchronization circuit and the like and the output from the chrominance signal processing circuit 9 is applied to a color demodulator 10. The color demodulator 10 comprises a double balance demodulating circuit and the like, so that three color difference signals are obtained based on the given chrominance signal. The luminance signal obtained from the luminance signal amplifier 6 and the color difference signals obtained from the color demodulator 10 are applied to a matrix circuit 11. The matrix circuit 11 is responsive to the given luminance signal and the color difference signals to provide three color primary signals, which are applied to a cathode ray tube. Since the above described structure of a conventional color television receiver is well-known to those skilled in the art, a more detailed description thereof will be omitted. However, it is pointed out that a point of interest to the present invention is that the delay line 5 is inserted in the luminance signal circuit 4. The present invention is directed to an improvement in a video signal processing circuit for enabling omission of such delay line 5. Therefore, delay characteristics at various portion of the FIG. 1 diagram will be described with reference to the associated figures.
FIG. 2 shows graphs showing signal delay characteristics at various portions of the FIG. 1 diagram. The solid line in the graph (A) in FIG. 2 shows a delay characteristic of a transmitter end. The delay characteristic of such transmitter end is determined in advance as a standard system so as to be adaptable to an average delay characteristic of possible typical color television receivers. On the other hand, receivers are adapted such that the delay characteristic at the transmitter end is compensated by a delay characteristic involved in the circuit portion from the tuner 1 to the video buffer amplifier 3 of the receiver. To that end, the total phase characteristic from the tuner 1 to the video intermediate frequency circuit 2 is selected to exhibit a delay characteristic as shown by the dotted line in (A) in FIG. 2. Accordingly, the delay characteristic at the output terminal of the video buffer amplifier 3 becomes substantially flat with respect to the frequency, as shown as (B) in FIG. 2.
On the other hand, the frequency/amplitude characteristic in the chrominance signal frequency band in the video buffer amplifier 3 is somewhat asymmetrical with respect to the color subcarrier frequency 3.58 MHz as shown as (A) in FIG. 3. Therefore, the frequency/amplitude characteristic of the bandpass filter 8 included in the chrominance signal circuit 7 is selected to attain a normal frequency/amplitude characteristic as shown as (C) in FIG. 3 by correcting the frequency/amplitude characteristic shown as (A) in FIG. 3. Therefore, the resonance point or the central frequency (f.sub.0) of the bandpass filter 8 is set to be considerably higher than the color subcarrier frequency (3.58 MHz). Accordingly, the delay characteristic in the chrominance signal frequency band in the bandpass filter 8 is not flat with respect to the frequency as shown as (C) in FIG. 2.
Thus, the delay characteristic of the bandpass filter 8 is as shown as (C) in FIG. 2 while the overall delay characteristic from the transmitter end up to the video buffer amplifier 3 is as shown as (B) in FIG. 2 and therefore the delay characteristic at the output end of the bandpass filter 8 becomes ultimately as shown as (D) in FIG. 2. The fact that the delay characteristic is inclined in the chrominance signal frequency band causes the amplitudes and the phases of both side bands of the chrominance signal to be different, which causes color distortion on the occasion of demodulation by the color demodulator 10. In case of such delay characteristic, it follows that the chrominance signal transferred through the chrominance signal circuit 7 is delayed in time as compared with the luminance signal transferred through the luminance signal circuit 4.
The color demodulator 10 comprises a double balance demodulator 10 as shown in FIG. 4, for example. A load resistor R1 of the double balance demodulator 10a is selected to be approximately 10k.OMEGA. and the load resistor R1 is shunted by a highpassing capacitor C1 of say 5 to 15 pF. The load resistor R1 may be considered as also shunted by a stray capacitance Cs of say 10 pF. A low pass filter 10b comprising an RC circuit is connected to the output signal circuit of the double balance demodulator 10a. A low pass filter stage is constituted by the load resistor R1, the highpassing capacitor C1, the stray capacitance Cs and the low pass filter 10b for removing a color subcarrier wave component of 3.58 MHz included in the chrominance signal. Therefore, the demodulated output at the color demodulator 10, i.e. the color difference signals are also subjected to time distortion at the low pass filter stage. More specifically, the delay characteristic at the low pass filter stage of the color demodulator 10 is as shown as (E) in FIG. 2. Therefore, the total delay characteristic from the transmitter end up to the low pass filter stage becomes ultimately as shown as (F) in FIG. 2. Meanwhile, it is pointed out that the characteristics shown as (E) and (F) in FIG. 2 show delay characteristics at the frequency band (0 to 500 kHZ) after color demodulation. As seen from the characteristics shown as (F) and (B) in FIG. 2, even if the luminance signal and the color difference signals as such are applied to the matrix circuit 11, the phases of the color difference signals and the luminance signal do not coincide with each other. As a result, it follows that proper color synthesization cannot be attained in the matrix circuit 11.
Therefore, conventionally a delay line 5 was necessarily inserted between the video buffer amplifier 3 and the luminance signal amplifier 6. By selecting the delay characteristic of such delay line 5 as shown as (G) in FIG. 2, the delay characteristic at the output terminal of the luminance signal amplifier 6 becomes as shown as (H) in FIG. 2. As a result, the phase of the luminance signal transferred through the luminance signal circuit 4 and the phase of the chrominance signal transferred through the chrominance signal circuit 7 come to coincide with each other, whereby proper color synthesization can be attained in the matrix circuit 11.
Meanwhile, the reason why a portion of the delay characteristic is shown by the dotted line as (H) in FIG. 2 will be described in the following. More specifically, although not shown, the luminance signal circuit 4 is provided with a trap circuit or a filter for removing the color subcarrier component at the stage before the luminance signal amplifier 6, for example. Such trap circuit is aimed to remove the so-called dot interference. Therefore, a component corresponding to the color subcarrier frequency is trapped by the trap circuit or the filter and accordingly the characteristic of the luminance signal is partially degraded accordingly.
For the above described reasons, conventional color television receivers necessarily employed a delay line in the luminance signal circuit. Furthermore, since the delay characteristic at the low pass filter stage included in the color demodulator 10 is not flat with respect to the frequency, as shown as (E) in FIG. 2, a disadvantage was encountered that although the phases of the color difference signals and the luminance signal approximately coincide with each other at a lower frequency region such as 0 to 200 kHZ both do not coincide with each other in a higher frequency region such as 200 to 500 kHz.